1. Field of the Invention
The present invention relates to a method for producing a ceramic tube and more particularly, to a method for producing a thin-walled ceramic tube having excellent dimensional precision.
2. Description of the Prior Art
Forming methods for ceramic articles are generally classified into pressing, casting and extruding and the best one of those methods is selected depending upon the shape of the articles and the required properties of the articles.
The forming of tube-shaped articles is generally accomplished by extruding. However, when it is required that the tube shaped articles have a thin wall and high dimension precisional isostatic pressing is utilized. Further, the isostatic pressing is typically combined with a mechancial finishing operation to reduce the thickness of the walls of the article.
As a typical example of an article in which the thin wall thickness and high dimensional precision are required, there is a translucent alumina tube for a high pressure sodium lamp which can be used as a light source. This translucent alumina tube is used as an envelope of a lamp, so that the light transmission of the tube is important. Hence, the wall of the tube must be thin. Furthermore, in order to maintain the gas-tightness of a sealed portion between metal electrodes and the tube and constantly maintain the lamp properties (lamp electric voltage, lamp output, stability of color tone), high dimensional precision is required.
For example, as the dimension specification of an envelope for a high pressure sodium lamp of 400 W, the following factors are required.
Outer diameter .phi.9.5.+-.0.1 mm PA1 Length 113.+-.1 mm PA1 Wall thickness 0.85.+-.0.1 mm PA1 Straightness less than 0.05 mm per 100 mm
When a general extruding technique is used to produce a tube-shaped article, for which such a dimension specification is necessary, the shaped body readily plastically deforms immediately after the extruding thereof and strain results in the axial direction of the tube. Hence, it has been difficult to obtain shaped articles having the above dimension specifications. Thus, the translucent alumina tube has been produced as follows. A tube having a somewhat larger thickness than the dimension specification is formed by isostatic pressing. The thus formed body is subjected to a working for abrading the outer surface to finish the body into the given thickness and outer diameter size and then the shaped body is fired. But, in this method, some type of subsequent machining is needed after the the body is isostatically pressed. Hence, the number of processing steps increases and further reusing the raw material powder abraded by the working of the outer circumference is difficult because of an admixture of impurities and the expensive alumina powder having a high purity is wastefully lost. Therefore, this process is uneconomic. Furthermore, in the pressing method, the shaped bodies are singly formed under a high pressure press, so that the productivity is poor and the rubber mold for the shaping to be used and the component (valves) of the isostatic press machine to which a high pressure is subjected, wear out rapidly resulting in this process also being uneconomical.